docs: def _maybe_set_message_age(self):
          """If timestamp is set and the relative age is > 0, record age of the
          message coming in
  
          """
          if self._message.properties.timestamp:
              message_age = float(
                  max(self._message.properties.timestamp, time.time()) -
                  self._message.properties.timestamp)
              if message_age > 0:
                  self.measurement.add_duration(
                      self.message_age_key(), message_age)